By Robert Bazell, NBC Chief science correspondent
The possible connection between thimerosal, a mercury preservative that was widely used in childhood vaccines through the 1990s, and autism, the common developmental disorder, is a difficult story to report. Autism leaves many children unable to communicate with the world, including their parents. Often the children are disabled in many ways. Caring for an autistic child is one of the greatest burdens any parent could face. Because mercury can indeed be a poison to the nerves and because children are often diagnosed at the time they get vaccinated, it is easy to understand why many parents would make the connection.
A federal study, out today in the New England Journal of Medicine, looked at the connection between a mercury-based preservative and children's vaccine. It does not directly concern autism, but it is very closely related to the ongoing dispute. Scientists from the Centers for Disease Control headed the effort where 1,047 children, aged 7 to 10 years, underwent a battery of tests measuring language, verbal skills, intelligence and fine motor coordination. Because the children were enrolled in large health insurance groups the scientists had good data on how much themerosal they had been exposed to earlier in life. For the vast majority of measurements the scientists did not find any correlations between thimerosal exposure and these neurophysiologic outcomes.
Many scientists outside the study say it would be difficult to see how thimerosal could cause something as severe as autism without causing huge changes in these measurable parameters.
You can see the study itself at content.nejm.org.
Two things to note: This is certainly not the end of this argument. Until researchers truly understand the cause of autism, no hypothesis can be ruled out. And parents who are now getting their children immunized should realize that, with the exception of some formulations of flu vaccine, there is no more than the smallest trace amounts in thimerosal remaining in any childhood vaccine